The present invention relates to the detection of unbalanced load current in alternating current power systems having a plurality of power sources operating in parallel and more particularly to a circuit for demodulating the real and reactive components of unbalanced load current.
AC electric power systems are usually connected in parallel to increase total system rating or in certain cases such as airborne power systems, to increase reliability. In order to further improve reliability and to maximize efficiency, it is generally desired that the total system load be divided equally among the paralleled generators. Load division among AC generators is accomplished by controlling, in a closed loop manner, individual generator voltages and phase angles. The exact method of control is dependent upon the impedances between the paralleled generator Thevenin voltages. For example, if the impedance between sources is primarily resistive, then a difference in Thevenin voltage magnitude will create an unbalance in real source power and a phase angle unbalance will create an unbalance in reactive source power. This impedance between sources must include the source or Thevenin impedance, the feeder bus impedance and parallel tie bus impedances.
In most AC power systems, the impedance between sources is primarily inductive. For this condition, voltage unbalance will create an unbalanced reactive source power and phase angle errors will cause an unbalance in real source power. However, exactly balanced Thevenin voltages and phase angles will not insure equal load division for all conditions. For example, a parallel tie bus with nonzero impedance which has unsymmetrically located load taps can cause an unbalance in impedance between sources and the load tap point. This can result in unbalanced source currents. Other variables such as manufacturing tolerances on the Thevenin source impedance can cause excessive current unbalance in a parallel system with otherwise matched voltage and phase angles. In a variable speed constant frequency (VSCF) aircraft power system, the source power is generated through an electronic power converter or inverter. These systems can be easily and accurately synchronized in phase with a common reference input signal. However, for the reasons mentioned above, this is not sufficient to eliminate the need for a real load division control loop. Since the VSCF source impedance is primarily inductive at fundamental frequency, the real load division loop must provide a means to alter the phase angle of the Thevenin voltages. Reactive load division is controlled through Thevenin voltage magnitude control.
Parallel alternating current systems including means for detecting unbalanced load current flow are disclosed in U.S. Pat. No. 3,294,976, issued Dec. 27, 1966 to L. L. Tipton, et al. and U.S. Pat. No. 3,748,493, issued July 24, 1973 to Billings, et al. These patents provide background information for the present invention and are hereby incorporated by reference. U.S. Pat. No. 3,879,670, issued Apr. 22, 1975 to Fox, discloses a synchronous demodulator circuit suitable for use as a detector to establish the amplitude and polarity of an unbalanced reactive load current in a parallel AC power system and is hereby incorporated by reference. The present invention performs the required demodulation in a manner similar to that disclosed in U.S. Pat. No. 3,879,670 with a substantially less complex circuit.